This week’s Republican presidential primaries in Maryland, Washington DC and Wisconsin are shaping up to be strong results for Mitt Romney and will add more weight to calls for his rivals to drop out.

In the past week, Republican young guns senator Marco Rubio and representative Paul Ryan, and old stagers president George HW Bush and senate minority leader Mitch McConnell have joined the chorus saying it is time for the party to get behind Mr Romney.

The trouble is neither Rick Santorum, Newt Gingrich or Ron Paul seems to be listening.

Last week Mr Gingrich was told by his principal financial-backer, casino billionaire Sheldon Adelson, there would be no more $US10,000,000 cheques for Newt's SuperPAC.

Mr Gingrich sacked his campaign manager and more than a third of his paid staff, yet vowed to stay in the race until the convention in August. He no longer expects to win primaries and delegates, just a future electability argument at a brokered convention.

Mr Santorum meanwhile is being outspent 10-to-1 by Mr Romney in Wisconsin, and has slipped 15 per cent behind him in National polls and trails by more than 300 delegates. But he too sees a path to the nomination whereby he makes up some ground in large winner-takes-all states like Texas in May.

And then there is Mr Paul, who has not won a single state primary or caucus and retains a veneer of a campaign in order to further his libertarian cause.

As former Romney advisor Steve Lombardo told me recently on Planet America, because Mr Santorum and Mr Gingrich are both out of elected office, and Mr Paul is effectively a party of one, there is nobody who can tap them on the shoulder and say it's time to get out.

Sometimes presidential candidates can be like old boxers, bloodied, beaten, absorbing blow after blow, but still swinging out of habit.

That's when they need someone in their corner prepared to throw in the towel before permanent damage is done.

Mr Adelson may have effectively thrown in the towel for Mr Gingrich, even as Newt continues to wander around the ring, but Mr Santorum and Mr Paul have a lot more small donors prepared to keep propping them up.